Add existence check (file protocol) in URLImageDescriptor.getFilePath()

The URLImageDescriptor.getFilePath() method currently returns a path for
a given URL using the file protocol even if the file does not exist.
This conflicts with the method's contract and breaks consumers (such as
the scaled image data retrieval in SWT's DPIUtil).

With this change, the method properly checks for the existence of a file
at the given location and returns null in case it does not exist,
according to the method's contract. A test case is added to demonstrate
the bug and detect regressions.

bundles/org.eclipse.jface/src/org/eclipse/jface/resource/URLImageDescriptor.java

@@ -22,6 +22,8 @@
 import java.io.InputStream;
 import java.net.MalformedURLException;
 import java.net.URL;
+import java.nio.file.Files;
+import java.nio.file.Path;
 
 import org.eclipse.core.runtime.FileLocator;
 import org.eclipse.core.runtime.IAdaptable;
@@ -267,8 +269,12 @@ private static String getFilePath(URL url, boolean logIOException) {
 				url = platformURL;
 			}
 			URL locatedURL = FileLocator.toFileURL(url);
-			if (FILE_PROTOCOL.equalsIgnoreCase(locatedURL.getProtocol()))
-				return IPath.fromOSString(locatedURL.getPath()).toOSString();
+			if (FILE_PROTOCOL.equalsIgnoreCase(locatedURL.getProtocol())) {
+				String filePath = IPath.fromOSString(locatedURL.getPath()).toOSString();
+				if (Files.exists(Path.of(filePath))) {
+					return filePath;
+				}
+			}
 			return null;
 		} catch (IOException e) {
 			if (logIOException) {

tests/org.eclipse.jface.tests/src/org/eclipse/jface/tests/images/UrlImageDescriptorTest.java

@@ -21,6 +21,7 @@
 import static org.junit.Assert.assertNotSame;
 import static org.junit.Assert.assertNull;
 
+import java.io.IOException;
 import java.net.URL;
 
 import org.eclipse.core.runtime.Adapters;
@@ -29,10 +30,15 @@
 import org.eclipse.swt.graphics.Image;
 import org.eclipse.swt.graphics.ImageData;
 import org.eclipse.swt.graphics.ImageFileNameProvider;
+import org.junit.Rule;
 import org.junit.Test;
+import org.junit.rules.TemporaryFolder;
 
 public class UrlImageDescriptorTest {
 
+	@Rule
+	public TemporaryFolder tempFolder = new TemporaryFolder();
+
 	/**
 	 * Test that individually created images of a given descriptor are not equal
 	 * (See issue #682).
@@ -114,6 +120,24 @@ public void testImageFileNameProviderGetxName() {
 		assertNull("URLImageDescriptor's ImageFileNameProvider does return a @1.5x path", imagePath150);
 	}
 
+	@Test
+	public void testImageFileNameProviderGetxName_forFileURL() throws IOException {
+		URL imageFileURL = tempFolder.newFile("image.png").toURI().toURL();
+		tempFolder.newFile("image@2x.png");
+		ImageDescriptor descriptor = ImageDescriptor.createFromURL(imageFileURL);
+
+		ImageFileNameProvider fileNameProvider = Adapters.adapt(descriptor, ImageFileNameProvider.class);
+		assertNotNull("URLImageDescriptor does not adapt to ImageFileNameProvider", fileNameProvider);
+		String imagePath100 = fileNameProvider.getImagePath(100);
+		assertNotNull("URLImageDescriptor ImageFileNameProvider does not return the 100% path", imagePath100);
+		assertEquals(IPath.fromOSString(imagePath100).lastSegment(), "image.png");
+		String imagePath200 = fileNameProvider.getImagePath(200);
+		assertNotNull("URLImageDescriptor ImageFileNameProvider does not return the @2x path", imagePath200);
+		assertEquals(IPath.fromOSString(imagePath200).lastSegment(), "image@2x.png");
+		String imagePath150 = fileNameProvider.getImagePath(150);
+		assertNull("URLImageDescriptor's ImageFileNameProvider does return a @1.5x path", imagePath150);
+	}
+
 	@Test
 	public void testAdaptToURL() {
 		ImageDescriptor descriptor = ImageDescriptor